Generally, there are three types of golf balls in the market, namely one-piece, two-piece and wound. One-piece balls are molded from a homogeneous mass of material, while two-piece balls are made by molding a cover about a solid core. Wound balls are made by molding a cover about a wound core. A wound core is prepared by winding thin elastic thread about a center.
Golf ball cores, whether wound or solid, typically measure from 1.4 to 1.6 inches (3.5 to 4.1 cm) in diameter. The cover is molded about the core to form a golf ball having the minimum United States Golf Association (USGA) specified diameter of 1.68 inches (4.3 cm). Typically, the cover has a thickness of about 0.04 inches (0.1 cm).
Conventionally, both two-piece and wound golf balls are made by molding covers about cores in one of two ways: by injection molding fluid cover stock material around a core which is held in a retractable pin mold; or by compression molding preformed half-shells about the core. The preformed half-shells are formed by injecting fluid cover stock material into half-shell molds and solidifying the cover stock material into a half-shell shape.
Balata was the standard cover stock material until the middle 1960's when E. I. DuPont de Nemours and Co. discovered a new species of resins known as ionomer resins. These resins are sold under the trademark SURLYN.TM. and, to a large extent, have replaced balata as a cover stock material. Chemically, these ionomer resins are a copolymer of an olefin and an alpha, beta ethylenically unsaturated carboxylic acid with 10-90% of the carboxylic acid groups being neutralized by a metal ion. See U.S. Pat. No. 3,264,272. Today, the only commercially available ionomer resins are copolymers of ethylene and methacrylic or acrylic acid. These ionomer resins are distinguished by the type of metal ion, the amount of acid, and the degree of neutralization.
Dunlop Rubber Company obtained the first patent on the use of Surlyn for the cover of a golf ball, see U.S. Pat. No. 3,454,280 issued Jul. 8, 1969. Since then, there have been a number of disclosures on the use of these ionomer resins in the cover composition of a golf ball. See, for example, U.S. Pat. No. 3,819,768 issued Jun. 25, 1974; U.S. Pat. No. 4,323,247 issued Apr. 6, 1982; U.S. Pat. No. 4,526,375 issued Jul. 2, 1985; U.S. Pat. No. 4,884,814 issued Dec. 3, 1989; and U.S. Pat. No. 4,911,451 issued Mar. 27, 1990.
In November 1986 DuPont introduced a sodium and zinc ionomer resin having a low flexural modulus and suggested using and blending the same with other ionomer resins for making a golf ball cover. Golf ball covers made from these low flexural modulus ionomer resins have good shear resistance but very low velocity.
In December of 1986, DuPont introduced a lithium ionomer resin which was a copolymer of ethylene and methacrylic acid. These lithium ionomer resins have a very high flexural modulus, typically about 60,000 psi (415 MPa). DuPont suggested that lithium ionomer resins could be used to produce a golf ball cover which would be more cut resistant and harder than a cover made with either sodium or zinc ionomer resins. DuPont also suggested that a golf ball having a cover made from a lithium ionomer resin would go farther, have a higher coefficient of restitution and be less prone to cutting (i.e., more durable) than a golf ball made from other known ionomer resins such as sodium and zinc ionomer resins and blends thereof. DuPont further suggested that lithium ionomer resins could be used in blends with other ionomer resins where they can impart better cut resistance to those other resins.
The United States Golf Association (USGA) has promulgated a rule that no golf ball shall have an initial velocity that exceeds 255 feet (78 m) per second, i.e., 250 feet (76 m) per second with a 2% tolerance. Golf balls with covers made from ionomer resins with low flexural modulus are woefully below this maximum and, as should be appreciated, all golf ball manufacturers strive to come as close as possible to this limit.
For many years, the ball of choice among advanced amateur players and tour professionals has been the balata covered wound ball. This ball is favored because it achieves spin rates sufficient to enable the player to more precisely control ball direction and distance, particularly on shorter approach shots. Balata covered wound balls, however, are expensive.
In order to approximate the characteristics of balata covered wound balls at lower cost, the art has developed two piece balls of varying cover compositions. Despite numerous attempts to replicate the performance of balata covered wound balls, two piece balls generally suffer from low spin rates which makes them difficult to control near the greens. Also, two piece balls tend to have relatively poor click and feel.
A need therefore exists for a two piece ball which has spin rates approximating those of balata covered wound balls.